The Fischer-Tropsch process is well known in the art. Synthesis gas, a mixture of hydrogen and carbon monoxide, is converted over a catalyst usually comprising a Group VIII metal or metal compound at elevated temperature and usually elevated pressure into mainly paraffinic and/or olefinic hydrocarbons and water. Depending on the reaction conditions (temperature, pressure, catalyst, H2/CO ratio, GHSV etc.) the product properties (e.g. the C5+ selectivity, the olefin content, the oxygenate content etc.) may vary. At the present moment there is a clear interest in the use of cobalt based catalyst at a temperature between 180 and 270° C. to make mainly very heavy paraffins comprising a major amount of normally solid hydrocarbons. In such Fischer-Tropsch processes substantial amounts of detergent hydrocarbons are produced, i.e. compounds having suitably 9 to 18 carbon atoms, preferably 10 to 17 carbon atoms.
The preparation of detergents, especially biodegradable detergents, from linear olefins prepared in a Fischer-Tropsch process has been described in the literature. For instance, in ACS Symp. Series No. 238, 18-33 (191 ACS Nat. Meeting Div. Pet. Chem. Symp. New York, 13-18 Apr. 1986) it has been described that C9-C15 cuts of low and high temperature Fischer-Tropsch processes are suitable feedstocks in the alkylation of benzene to prepare alkylbenzenes, followed by sulfonation and neutralization to convert the alkylbenzenes into alkylbenzene sulfonates. The direct products of these Fischer-Tropsch processes, using iron based catalysts, comprise rather large amounts of olefins. For instance, the high temperature process results in a product comprising about 70% olefins (60% straight chain product), the low temperature process results in about 25% olefins (linearity 93%). Also, U.S. Pat. No. 3,674,885 describes the use of paraffin-olefin mixtures synthesized in a Fischer-Tropsch process in the alkylation of benzene. The paraffins are separated from the alkylation mixture and are recycled to a chlorination unit from which the paraffin-chloroparaffin effluent mixture is combined with the fresh Fischer-Tropsch olefin-paraffin mixture and the combined feeds are used to alkylate the benzene. Detergents may also be made directly from paraffins as described in WO 99/59942.
There exists a clear commercial demand for linear hydrocarbons for the preparation of detergents. In general, the more linear the product, the higher the demand. See for instance U.S. Pat. No. 6,392,109 column 1, lines 12 and 13, and lines 28 to 31, clearly indicating that linear detergent hydrocarbon are preferred over branched detergent hydrocarbons. Thus, there is a clear need for detergent hydrocarbons with a (very) low amount of branched hydrocarbons. It has now been found that when carrying out a Fischer-Tropsch reaction using a cobalt based catalyst the amount of branched hydrocarbons decreases at higher pressures. Thus, when using the same reaction temperature, at higher pressures less branching occurs, at lower pressure more branching occurs.